This invention relates generally to radioactivity well logging methods and apparatus for investigating subsurface earth formations by neutron logging techniques, and more particularly, to a new and improved methods of determining oil and water bearing formations of the earth traversed by a well bore.
In the prior art of neutron logging, a fast neutron source on a logging tool irradiates the formations surrounding an open or uncased borehole. The resulting secondary radiation within the borehole is measured by two thermal neutron detectors longitudinally spaced from one another and from the source. By proper spacing of the detectors and the source, formation porosity can be determined from the counts generated by the detectors. One such system is illustrated in U.S. Pat. No. 4,004,147 issued to L. S. Allen.
A two group neutron diffusion theory describes the secondary radiation as being epithermal and thermal neutrons. The thermal neutron parameter of the formation typically is related to porosity but is strongly affected by the total macroscopic absorption cross section of the formation, that is, both the rock matrix and the fluids within the formation.
The macroscopic absorption cross section of the formation fluid is affected by the salinity of the fluid and is significantly reduced when the pore spaces of the formation contain salt water rather than oil. The chlorine present in the salt water has a large macroscopic absorption cross section for thermal neutrons and, consequently, reduces the number of thermal neutrons returning to the borehole as secondary radiation.
This present invention is not concerned with determination of porosity but rather producing a direct reading response identifying movable oil and movable water by a pair of special low atmosphere gas thermal neutron detectors in a logging tool and employing a new and different system of operation of the setup or calibration of the two neutron detectors outputs to the recording device.